P3.1 - Static and charge

• static electricity:

  All matter contains charge - atoms contain positive protons and negative electrons. Most matter contains an equal number of positive and negative charges, so their effects cancel each other out (the matter has zero net charge and is neutral). But in some situations charge can build up on objects this is static electricity
• How static electricity works:

  When two materials are rubbed together, electrons are transferred from one to the other.

  If the materials are conductors, the electrons will flow back into or out of them, so they stay neutral. But if the materials are insulators, electrons can't flow, so a positive static charge is left on the object that lost electrons and a negative static charge is left on the object that gained electrons.
• Static electricity:

  It's always negative charges (electrons) that move.The direction of electron transfer depends on the materials: If enough charge builds up, it can suddenly move, causing sparks or shocks.

  A) polythene rod

  B) from the duster

  C) acetate rod

  D) from the rod

  
• Like and opposite charges:

  Electrically charged objects exert a force on one another.

  These forces get weaker the further apart the two objects are.

  Things with opposite electric charges attract each other, things with the same electric charge repel. The force between two charged objects is known as electrostatic attraction (if they attract each other),

  or electrostatic repulsion (if they repel). It's a non-contact force the objects don't need to touch.

  
• Testing whether an object is charged:

  Electrically charged objects attract small neutral objects placed near them. E.g. if you hold a charged rod above small scraps of paper the scraps will 'jump' towards it.

  This happens because the charged rod induces a charge in the paper - if rod is positively charged, it attracts electrons in paper towards it, and if it's negatively charged it repels electrons. This gives surface of paper near rod an opposite charge to rod, so rod and paper are attracted together.

  A) negative

  B) repelled

  C) positive

  D) attracted

  
• You can also test if a rod is charged by holding it near a stream of water from a tap. The rod will induce a charge in the water, so the stream will be attracted to the rod and bend towards it.
• Gold leaf electroscope experiment:

  You can test for charge using a gold leaf electroscope:

  • If a negatively charged insulator touches the zinc plate,

  some of its charge is transferred to the electroscope, and conducted down to the metal stem and gold leaf. This negatively charges both the stem and the gold leaf, which repel each other. This makes the gold leaf rise. If you touch the plate with a positively charged insulator, electrons flow into it from the plate, stem and leaf. Again, the stem and leaf will have the same charge and the leaf will rise.

  
• Electric fields lines:

  Charged objects have electric fields, which you can show with electric field lines.

  Electric field lines go from positive to negative. They're always at a right angle to the surface of the object at the point where they touch the surface.

  The closer together the field lines are, the stronger the field, and the stronger the force a charged object in the field experiences.
• For charged spheres, like the image, field lines get further apart the further from the sphere you are, so the force another charged object feels due to an electric field decreases with distance.

  A) uniformly

  B) positive

  C) +

  D) negative

  E) -

  
• When the electric fields around two charged objects interact, an electrostatic force is produced.

  If the field lines between the charged objects point in the same direction, the field lines 'join up' and the objects are attracted to each other. This happens when the two charges are opposite. If the charges are free to move, the field lines will straighten and shorten as the charges move together.

  A) attraction

  B) -

  C) +

  
• When the field lines between the charged objects point in opposite directions, the field lines 'push against' each other and the objects repel each other. This happens when the two charges are the same type.

  A) repulsion

  B) repulsion

  C) +

  D) +

  E) -

  F) -

  
• Current:

  Current is the rate of flow of electric charge (electrons) around the circuit. Current will only flow through an electrical component if there is a potential difference across that component, and if the circuit is complete (closed). Unit: ampere, A.
• Potential Difference:

  Potential Difference is the driving force that pushes the current round. Unit: volt, V.
• Generally speaking, the higher the potential difference across a given component, the higher the current will be. And the greater the resistance of a component, the smaller the current that flows (for a given potential difference across the component).
• Resistance


  Resistance is a measure of how easily charge can flow. Unit: ohm, Ω.
• In a single closed loop (like the one in the diagram) the current will have the same value at any point.

  A) potential difference

  B) current flows

  C) resistance

  D) opposes

  E) flow

  
• Current is the rate of flow of charge. If a current (I) flows past a point in a circuit for a length of time (t), then the charge (Q) that has passed this point is given by this formula: charge = current × time

  To use this formula, you need current in amperes, A, charge in coulombs, C and time in seconds, s. More charge passes around the circuit in a given time when a greater current flows.
• The potential difference (V) is the energy transferred (E) per coulomb of charge (Q) that passes between two points in an electrical circuit. You can calculate energy transferred: energy transferred (Joules, J) = charge (c) x potential difference (v)

  So, the potential difference (p.d.) across an electrical component is the amount of energy transferred by that electrical component (e.g. to the kinetic energy store of a motor) per unit of charge. Potential difference is sometimes called voltage. They're the same thing.